Purvi Patel, a 33-year-old woman from Indiana, was convicted Tuesday night on charges stemming from a pregnancy that ended in tragedy. Patel was accused by prosecutors of illegally inducing an abortion by taking pills ordered online from Hong Kong, thus committing what’s referred to in Indiana state law as “feticide,” then failing to properly care for her baby during the first moments of its life, essentially allowing it to die. Police got involved in the case after Patel arrived at a hospital bleeding from her vagina; she initially denied having given birth, but later told doctors she had delivered a stillborn fetus at home, then placed the body in a dumpster.

The two charges against Patel—feticide and felony child neglect—appeared to contradict each other: If Patel killed the fetus with pills while it was still in the womb, that would suggest there was nothing she could do to save it once it was born. Nevertheless, a jury found Patel guilty of both crimes, meaning she could be facing up to 70 years in prison.

The apparent paradox at the heart of the charges against Patel is one of the reasons her case received widespread attention Wednesday. When I asked the St. Joseph County, Indiana, prosecutor, Ken Cotter, to explain it, he pointed out that according to Indiana law, a person can be guilty of feticide even if the fetus in question survives, as long as a deliberate attempt was made to “terminate” the pregnancy “with an intention other than to produce a live birth or to remove a dead fetus.” (The statute includes an exemption for legal abortions.) The prosecution contended that Patel intended to kill the fetus by taking the pills (feticide) and when she failed, allowed the living fetus to die (felony child neglect).

There’s another reason Patel’s case deserves scrutiny. It has to do with how the prosecution went about establishing the fetus’s condition upon birth. At the center of its presentation was a method that involved removing the fetus’s lungs and placing them in a container of liquid in order to see if they would float. The theory behind the method, which was developed during the 17th century but has been questioned by modern medical experts, holds that if a lung does float, it means the baby drew at least one breath of air before expiring, and that if it sinks, the fetus was already dead by the time it left the womb.

The procedure, known as the “lung float test” or the “the hydrostatic test,” was carried out in this case by forensic pathologist Joseph Prahlow, a past president of the National Association of Medical Examiners. Prahlow, who declined to comment for this article, testified in court that the lung of the Patel fetus did indeed float, suggesting it was born alive and that its mother could have taken steps to keep it that way. Prahlow supplemented the evidence from the float test with other findings, testifying that the lungs looked full of air when he removed the fetus’s chest plate during the autopsy, that the air sacs in the lung tissue looked expanded when he looked at them under a microscope, and that the weight of the lungs—approximately 21 grams—was consistent with a live birth. Prahlow also testified that, according to his analysis, blood had started flowing to the lungs, which would have only happened after the baby had taken a breath.

All those pieces of evidence surely played a part in convincing the jury in this case that Patel’s baby was born alive. But the lung float test stands out for its simplicity—a lung that floats means born alive, a lung that sinks means stillborn—and for how decisively it appears to answer one of the most complex questions that forensic pathologists face.

It’s far from clear, however, that the test can be trusted.

“It’s an absolutely discredited test,” said Gregory Davis, a professor of pathology and laboratory medicine at the University of Kentucky. “It boggles my mind that in the 21st century … this test is still being relied upon to determine whether a baby is born alive or dead.”

Davis is not the only forensic pathologist who believes the float test is unreliable. The most recent edition of Knight’s Forensic Pathology, a widely used textbook, says “there are too many recorded instances when control tests have shown that stillborn lungs may float and the lungs from undoubtedly live-born infants have sunk, to allow it to be used in testimony in a criminal trial.” The authors of another textbook, Essentials of Forensic Medicine, called the test “pointless” in 1984.

Davis, who is also the assistant state medical examiner for the commonwealth of Kentucky, said there are at least three reasons why a float test could yield inaccurate results, indicating the presence of air in the lungs even though the fetus never took a breath. The first is easiest to understand: If any attempt at resuscitation was made, either through mouth-to-mouth or chest compressions, that can introduce air into a lung, thus causing it to float even if the fetus was stillborn. The second has to do with decomposition: If the fetus has decomposed even a little bit, the lungs can fill with gas bubbles that would also result in the lung floating. Finally, Davis said, a fetus’s lungs can fill with air just by going through the vaginal canal, because pressure on the chest creates a “bellows effect.”

Despite these inherent flaws in the test, its use persists. In a 2006 case in Alabama, a medical examiner presented it as evidence that a baby had been suffocated by its mother. The woman spent nine months in jail before the case against her was dropped, thanks to forensics experts brought in by the defense, which showed that the fetus had suffered from pneumonia and was stillborn.

In Patel’s case, the medical examiner testified that there had been no decomposition, ruling that out as the cause of flotation. However, according to WSBT 22, a news network in Indiana, Patel told detectives in a taped interview shown in court that she had “opened the baby’s mouth and tried to resuscitate it,” which might explain why there would have been air in the fetus’s lungs. (Prahlow testified that Patel’s resuscitation attempts, as she described them, were so minimal that they wouldn’t have had any effect on the baby’s lungs.)

Ultimately, forensic pathology is a science of probabilities in which certainty is often elusive. As forensic pathologist Suzanna Dana, who serves on the editorial board of the American Journal of Forensic Medicine and Pathology, put it, “In some cases you end up not really knowing if the baby breathed on its own. Nothing’s 100 percent.”

The float test, however, has the appearance of clarity. As even the prosecutor in the Patel case put it, it “sounds like something you’d do in a sixth-grade laboratory test.” Its simplicity is its appeal, but it also has the potential to conceal the complexities inherent in any forensic analysis. That’s why, said Davis, it’s a dangerous thing to present to a jury. It’s also why prosecutors like using it.